walwriter.c

/*------------------------------------------------------------------------- * * walwriter.c * * The WAL writer background process is new as of Postgres 8.3. It attempts * to keep regular backends from having to write out (and fsync) WAL pages. * Also, it guarantees that transaction commit records that weren't synced * to disk immediately upon commit (ie, were "asynchronously committed") * will reach disk within a knowable time --- which, as it happens, is at * most three times the wal_writer_delay cycle time. * * Note that as with the bgwriter for shared buffers, regular backends are * still empowered to issue WAL writes and fsyncs when the walwriter doesn't * keep up. * * Because the walwriter's cycle is directly linked to the maximum delay * before async-commit transactions are guaranteed committed, it's probably * unwise to load additional functionality onto it. For instance, if you've * got a yen to create xlog segments further in advance, that'd be better done * in bgwriter than in walwriter. * * The walwriter is started by the postmaster as soon as the startup subprocess * finishes. It remains alive until the postmaster commands it to terminate. * Normal termination is by SIGTERM, which instructs the walwriter to exit(0). * Emergency termination is by SIGQUIT; like any backend, the walwriter will * simply abort and exit on SIGQUIT. * * If the walwriter exits unexpectedly, the postmaster treats that the same * as a backend crash: shared memory may be corrupted, so remaining backends * should be killed by SIGQUIT and then a recovery cycle started. * * * Portions Copyright (c) 1996-2009, PostgreSQL Global Development Group * * * IDENTIFICATION * $PostgreSQL$ * *------------------------------------------------------------------------- */#include "postgres.h"#include <signal.h>#include <sys/time.h>#include <time.h>#include <unistd.h>#include "access/xlog.h"#include "libpq/pqsignal.h"#include "miscadmin.h"#include "postmaster/walwriter.h"#include "storage/bufmgr.h"#include "storage/fd.h"#include "storage/ipc.h"#include "storage/lwlock.h"#include "storage/pmsignal.h"#include "storage/smgr.h"#include "utils/guc.h"#include "utils/hsearch.h"#include "utils/memutils.h"#include "utils/resowner.h"/* * GUC parameters */int WalWriterDelay = 200;
/* * Flags set by interrupt handlers for later service in the main loop. */staticvolatile sig_atomic_t got_SIGHUP = false;
staticvolatile sig_atomic_t shutdown_requested = false;
/* Signal handlers */staticvoid wal_quickdie(SIGNAL_ARGS);
staticvoid WalSigHupHandler(SIGNAL_ARGS);
staticvoid WalShutdownHandler(SIGNAL_ARGS);
/* * Main entry point for walwriter process * * This is invoked from BootstrapMain, which has already created the basic * execution environment, but not enabled signals yet. */void
WalWriterMain(void)
{
sigjmp_buf local_sigjmp_buf;
MemoryContext walwriter_context;
/* * If possible, make this process a group leader, so that the postmaster * can signal any child processes too. (walwriter probably never has any * child processes, but for consistency we make all postmaster child * processes do this.) */#ifdef HAVE_SETSIDif (setsid() < 0)
elog(FATAL, "setsid() failed: %m");
#endif/* * Properly accept or ignore signals the postmaster might send us * * We have no particular use for SIGINT at the moment, but seems * reasonable to treat like SIGTERM. */
pqsignal(SIGHUP, WalSigHupHandler); /* set flag to read config file */
pqsignal(SIGINT, WalShutdownHandler); /* request shutdown */
pqsignal(SIGTERM, WalShutdownHandler); /* request shutdown */
pqsignal(SIGQUIT, wal_quickdie); /* hard crash time */
pqsignal(SIGALRM, SIG_IGN);
pqsignal(SIGPIPE, SIG_IGN);
pqsignal(SIGUSR1, SIG_IGN); /* reserve for sinval */
pqsignal(SIGUSR2, SIG_IGN); /* not used *//* * Reset some signals that are accepted by postmaster but not here */
pqsignal(SIGCHLD, SIG_DFL);
pqsignal(SIGTTIN, SIG_DFL);
pqsignal(SIGTTOU, SIG_DFL);
pqsignal(SIGCONT, SIG_DFL);
pqsignal(SIGWINCH, SIG_DFL);
/* We allow SIGQUIT (quickdie) at all times */#ifdef HAVE_SIGPROCMASK sigdelset(&BlockSig, SIGQUIT);
#else BlockSig &= ~(sigmask(SIGQUIT));
#endif/* * Create a resource owner to keep track of our resources (not clear that * we need this, but may as well have one). */
CurrentResourceOwner = ResourceOwnerCreate(NULL, "Wal Writer");
/* * Create a memory context that we will do all our work in. We do this so * that we can reset the context during error recovery and thereby avoid * possible memory leaks. Formerly this code just ran in * TopMemoryContext, but resetting that would be a really bad idea. */
walwriter_context = AllocSetContextCreate(TopMemoryContext,
"Wal Writer",
ALLOCSET_DEFAULT_MINSIZE,
ALLOCSET_DEFAULT_INITSIZE,
ALLOCSET_DEFAULT_MAXSIZE);
MemoryContextSwitchTo(walwriter_context);
/* * If an exception is encountered, processing resumes here. * * This code is heavily based on bgwriter.c, q.v. */if (sigsetjmp(local_sigjmp_buf, 1) != 0)
{
/* Since not using PG_TRY, must reset error stack by hand */
error_context_stack = NULL;
/* Prevent interrupts while cleaning up */
HOLD_INTERRUPTS();
/* Report the error to the server log */
EmitErrorReport();
/* * These operations are really just a minimal subset of * AbortTransaction(). We don't have very many resources to worry * about in walwriter, but we do have LWLocks, and perhaps buffers? */
LWLockReleaseAll();
AbortBufferIO();
UnlockBuffers();
/* buffer pins are released here: */
ResourceOwnerRelease(CurrentResourceOwner,
RESOURCE_RELEASE_BEFORE_LOCKS,
false, true);
/* we needn't bother with the other ResourceOwnerRelease phases */
AtEOXact_Buffers(false);
AtEOXact_Files();
AtEOXact_HashTables(false);
/* * Now return to normal top-level context and clear ErrorContext for * next time. */
MemoryContextSwitchTo(walwriter_context);
FlushErrorState();
/* Flush any leaked data in the top-level context */
MemoryContextResetAndDeleteChildren(walwriter_context);
/* Now we can allow interrupts again */
RESUME_INTERRUPTS();
/* * Sleep at least 1 second after any error. A write error is likely * to be repeated, and we don't want to be filling the error logs as * fast as we can. */
pg_usleep(1000000L);
/* * Close all open files after any error. This is helpful on Windows, * where holding deleted files open causes various strange errors. * It's not clear we need it elsewhere, but shouldn't hurt. */
smgrcloseall();
}
/* We can now handle ereport(ERROR) */
PG_exception_stack = &local_sigjmp_buf;
/* * Unblock signals (they were blocked when the postmaster forked us) */
PG_SETMASK(&UnBlockSig);
/* * Loop forever */for (;;)
{
long udelay;
/* * Emergency bailout if postmaster has died. This is to avoid the * necessity for manual cleanup of all postmaster children. */if (!PostmasterIsAlive(true))
exit(1);
/* * Process any requests or signals received recently. */if (got_SIGHUP)
{
got_SIGHUP = false;
ProcessConfigFile(PGC_SIGHUP);
}
if (shutdown_requested)
{
/* Normal exit from the walwriter is here */
proc_exit(0); /* done */
}
/* * Do what we're here for... */
XLogBackgroundFlush();
/* * Delay until time to do something more, but fall out of delay * reasonably quickly if signaled. */
udelay = WalWriterDelay * 1000L;
while (udelay > 999999L)
{
if (got_SIGHUP || shutdown_requested)
break;
pg_usleep(1000000L);
udelay -= 1000000L;
}
if (!(got_SIGHUP || shutdown_requested))
pg_usleep(udelay);
}
}
/* -------------------------------- * signal handler routines * -------------------------------- *//* * wal_quickdie() occurs when signalled SIGQUIT by the postmaster. * * Some backend has bought the farm, * so we need to stop what we're doing and exit. */staticvoid
wal_quickdie(SIGNAL_ARGS)
{
PG_SETMASK(&BlockSig);
/* * DO NOT proc_exit() -- we're here because shared memory may be * corrupted, so we don't want to try to clean up our transaction. Just * nail the windows shut and get out of town. * * Note we do exit(2) not exit(0). This is to force the postmaster into a * system reset cycle if some idiot DBA sends a manual SIGQUIT to a random * backend. This is necessary precisely because we don't clean up our * shared memory state. */
exit(2);
}
/* SIGHUP: set flag to re-read config file at next convenient time */staticvoid
WalSigHupHandler(SIGNAL_ARGS)
{
got_SIGHUP = true;
}
/* SIGTERM: set flag to exit normally */staticvoid
WalShutdownHandler(SIGNAL_ARGS)
{
shutdown_requested = true;
}